The present invention relates to a shaped body which has been produced from a reaction mixture comprising zeolite, plasticizing agent and binder. The present invention furthermore relates to a process for the production of such a shaped body and to its use.
Zeolites are crystalline microporous materials of defined pore structure. These find diverse industrial applications in the form of fixed beds in bulk form, for example for drying gases, removal of volatile organic hydrocarbons from waste air streams and separation of hydrocarbon fractions.
A disadvantage of the use of fixed beds in bulk form of zeolite is, above all, the abraded material obtained during operation. This undesirable dust-like abraded material impairs functioning of fittings and apparatuses located downstream. Furthermore, when gases flow through, fixed beds in bulk form show pressure losses which are significantly greater compared with zeolitic honeycombed bodies and can vary during operation.
In order to be able to originally form zeolitic powders into shaped bodies, it is necessary to add to the powder, in a compounding step, additives which impart to the moulding composition a certain plasticity, which in turn is a prerequisite for the subsequent shaping process. After shaping has been concluded, it should be possible to remove the plasticizing agent from the form produced, such as, for example, an extrudate, as far as possible without residues, because even small amounts of foreign substances impair the sorptive and catalytic properties of the zeolitic shaped body produced. To obtain a dimensionally stable shaped body, it is furthermore necessary to add a binder to the moulding composition. During the subsequent calcining, this binder decomposes to form bridges of solid.
A process for the production of hard, break-proof catalysts from zeolites of the pentasil family is described, for example, in DE 32 31 498 A1. Tetramethyl orthosilicate (TMOS) is used here as the binder in an amount of 5%. The plasticizing agent is hydroxyethylcellulose, which is employed in an amount of 2%. After this mixture has been kneaded, it is transferred to an extruder and extruded to strands. After the shaped body produced in this way has been dried, it is calcined at temperatures in a range from 400xc2x0 C. to 800xc2x0 C. for 2 hours.
A disadvantage of the process described in this publication is, above all, that because of the high temperatures and the long calcining time, the production of the zeolitic shaped bodies is very cost-intensive. The fact that mixing of the moulding composition and extrusion are carried out in separate steps is also a contributing factor to this. The binder tetramethyl orthosilicate described in this publication furthermore is a small molecule which is deposited inside the pentasil zeolites, blocks these and therefore impairs their sorptive and catalytic properties.
DE 37 38 916 A1-describes molecular sieve shaped bodies of zeolites in which silica sol is primarily used as the binder, the SiO2 particles contained in this silica sol having a specific surface area, measured by the BET method, of 150 m2/g-400 m2/g. Moulding compositions of a mixture of silica sol with a mixture of ethyl esters of methylsilicic acids are furthermore described, the latter being added to the silica sol in small amounts. Lubricants which allow a fine rheological adjustment of the moulding composition recipe used are added to the moulding compositions. The molecular sieve shaped bodies comprise zeolites of the Y type or mordenite type. The molecular sieve shaped bodies are produced by extrusion, the moulding composition first being mixed and kneaded and then being transferred to the extruder in a second step. The calcining is carried out at 500xc2x0 C. to 800xc2x0 C.
It is a decided disadvantage of the molecular sieve shaped bodies described in DE 37 38 916 A1 that they are produced in a discontinuous process at very high temperatures. As a result, the production costs of the shaped bodies increase considerably. It is furthermore a disadvantage that in the mixture used, of silica sol with ethyl esters of methylsilicic acids, the silica sol is present in colloidal form. It is therefore not possible to form a continuous matrix. The solid binder can be distributed poorly compared with the binder present in liquid form. Finally, expensive lubricants are added.
Zeolitic shaped bodies produced by extrusion in the form of honeycombed bodies are not yet commercially obtainable.